Diesel motor vehicles which contain a common rail injection system are already known. In said systems, the rail pressure is one of the main parameters which influence the fuel injection quantity. For this reason, the presence of as stable a rail pressure as possible is an essential precondition for accurate metering of the fuel injection quantity.
In what is known as a VCV closed loop control system, the rail pressure is dependent on the cylinder filling of the high-pressure fuel pump. Non-uniform filling of the cylinders in a two cylinder pump or a three cylinder pump leads to pressure fluctuations in the rail. Non-uniform filling of this type of the cylinders can be ascribed, inter alia, to different volumetric flow characteristics of the inlet valves of the cylinders. The different volumetric flow characteristics of the inlet valves are caused, in particular, by different opening pressures of the inlet valves of the cylinders, which inlet valves are realized as an intake valve. The different opening pressures are to be ascribed, for example, to production-related different spring prestresses of the inlet valves and/or to undefined contact lines between the closing body and the valve seat of the inlet valves. Furthermore, the stated contact line of an inlet valve can change in the first operating hours of the inlet valve as a result of a deformation of the valve seat in an undesired manner.
FIG. 1 shows one example for the dependence of the cylinder filling on the opening pressure of the inlet valve. Here, the pressure difference dP in bar is shown along the ordinate and the fuel inlet quantity Q in liters per minute is shown along the abscissa. The curve K1 describes an inlet operation, in which the opening pressure corresponds to a pressure difference dP of 1.2 bar, and the curve K2 describes an inlet operation, in which the opening pressure corresponds to a pressure difference dP of 1.4 bar. It can be seen that, if a pressure difference dP of 1.5 bar is present, the inlet quantity at an opening pressure of the inlet valve of 1.2 bar is greater by ΔQ≈0.1 l/min than the inlet quantity at an opening pressure of the inlet valve of 1.4 bar. Furthermore, it can be seen from FIG. 1 that the volumetric flow characteristic of conventional inlet valves is linear, that is to say the change in the inlet quantity proceeds linearly with respect to a change in the pressure difference dP.
FIG. 2 shows a diagram, in which the delivery volume of the cylinders is shown as a function of time. Here, the conveying volume in liters is plotted along the ordinate and the time in seconds is plotted along the abscissa. The curve K3 with the continuous lines is assigned to a cylinder, the inlet valve of which has an opening pressure of 1.4 bar, and the curve K4 with the dashed lines is assigned to a cylinder, the inlet valve of which has an opening pressure of 1.2 bar. It can be seen that the delivery volume of both cylinders deviates by ΔQ≈0.02 liter per inlet operation.
The non-uniform filling described in the preceding text of the cylinders of a high-pressure fuel pump can lead in the extreme case to a failure of a cylinder. This means that a two or three cylinder pump operates like a one cylinder pump at very low inlet quantities.
The opening pressure of an inlet valve lies in the range between 1.2 and 1.7 bar. At an opening pressure which is lower than 1.2 bar, the risk increases that an air/liquid mixture is sucked through the intake valve into the compression chamber. As a result of the entrained, compressible air, no complete filling is achieved and the pressure pulses in the rail increase.
At an opening pressure which is greater than approximately 1.7 bar, the losses during the starting operation of the engine rise. Said losses manifest themselves in such a way that the filling of the compression chamber of the high-pressure pump is limited by late opening of the intake valves, as a result of which the starting time increases on account of reduced quantities or pressure availability.
In the context of the production of inlet valves of this type, said inlet valves are measured and divided into different classes. In practice, production failures of up to 50% occur with the current design.